In an injection molding machine, a die casting machine or the like, a pair of molds therein must be kept in a closed state when a molding product is being molded. In order to maintain the molds in the closed state, against a greater inner pressure produced within the molds, a mold clamping operation is necessary to press the pair of molds towards each other by strong forces from the opposite sides. For this reason, in a molding device in an injection molding machine or the like, a molds clamping device, for maintaining the molds in the closed state during the molding operation, is provided together with an injection device for injecting a raw molding material in a molten state.
These conventional known mold clamping devices include a direct pressure type mold clamping device having a hydraulic driving type mold clamping piston-cylinder mechanism by means of which a pair of molds are directly pressed, and a toggle type mold clamping device having a toggle mechanism by means of which a mold clamping force is retained. In these mold clamping devices, platens are provided to retain the number of the molds necessary for forming a cavity in which a molding product is formed, one of these platens being used as a stationary platen, and the other being used as a movable platen which is moved towards the stationary platen.
When clamping the molds, a greater pressing force is applied to the molds between the movable platen and the stationary platen by pressing the mold clamping piston constituting the mold clamping piston-cylinder mechanism directly or through the toggle mechanism against the back side of the movable platen.
In this case, in the conventional device, the stationary platen and the clamping cylinder constituting the mold clamping piston-cylinder mechanism are connected by a plurality of bars (usually four) parallel to each other, and these tie bars are used as a guide to move the movable platen.
These tie bars are arranged in positions so as to enclose the molds as a center and serve as a sliding guide for the movable platen. Furthermore, they have a function of forming a closed loop between the stationary platen and the clamping cylinder to retain interiorly the mold clamping force. This makes the tie bars indispensable elements for the mold clamping device using the conventional hydraulic driving type clamping piston-cylinder mechanism.
However, since the tie bars are made of costly special alloy steel to maintain the strength thereof, they bring about an increased cost in the clamping device and also an increased number of parts for the mold clamping device.
Furthermore, the existence of the tie bars in the outer periphery of the molds causes some interference with the tie bars in the exchanging operation of the molds, which results in reduced efficiency of operation. For this reason, an attempt was made to axially move one of the four tie bars to thereby facilitate the exchange of the molds (for example, refer to Utility Model Publication No.23656/1975 and Utility Model Publication No. 27966/1990); however such a method requires the special provision of a transfer mechanism for the tie bars, which makes the construction of the mold clamping device more complicated.
In addition, the provision of the hydraulic driving type cylinder-piston mechanism at the rear side of the movable platen makes the mold clamping device longer in the axial direction, which causes the longer longitudinal dimension of an injection molding machine, a die casting machine or the like. This, in turn, makes it necessary to provide a greater work space when the injection molding machine is installed in a factory, and therefore has a disadvantage in that the efficiency of the factory layout is decreased.
Moreover, the hydraulic driving type piston-cylinder mechanism is problematic in that it requires some countermeasures for preventing the deterioration of the work environment due to leakage of oil or oil catching on fire.
This invention has been made taking such points into consideration, and aims at providing a mold clamping device which enables the number of parts to be reduced and a smaller sized device to be produced.
A further object of the invention is to provide a mold clamping device which permits the exchanging operation of the molds to be easily carried out due to the fact that the tie bars are made unnecessary.
Another object of the invention is to provide a mold clamping device which permits the maintenance and control the device to be facilitated due to the fact that the hydraulic driving system is removed.
A further object of the invention is to provide a mold clamping device which enables the control of the mold clamping operation to be facilitated and the most suitable clamping operation to always be carried out.